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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Misery named the Japanese beetle, Popillia japonica


A horde of beetles can put a beating on trees, shrubs, and herbaceous plants. 


After a hiatus of many years, Japanese beetles seem to be making a comeback in our region. The first detection of this ravenous beetle in the United States was in 1916 in a plant nursery in New Jersey. The lore about Japanese beetles holds that in years of summer droughts, the numbers of Japanese beetles are reduced the following year due to poor survival of eggs and young grubs. For several years we have suffered or, perhaps, enjoyed the kinds of droughts that could really set these buggers back. Unfortunately, in my garden and in several others I recently visited, Japanese beetles seem to be making a comeback. Historically, July is a month of misery when Japanese beetles abound. However, in this unseasonably warm 2012, these mischief makers have arrived a few weeks early.

These six-legged gastronomes consume more than 300 kinds of trees, shrubs, vines, and herbaceous plants. Among their favorites are lindens, maples, apples, cherries, grapes, and, sadly, roses. In a series of careful studies, Dan Potter and his colleagues in Kentucky found that roses with large, light colored blossoms, particularly yellow or white, were more attractive to beetles than varieties with smaller, darker blossoms of red or orange. In the tree realm, researchers noted that lindens with densely hairy leaves were less preferred than scantily haired varieties. Maples with purple or deep red leaves were preferred over those with green leaves. A comprehensive listing of susceptible and resistant varieties is contained in the important reference by David Held at the treelink url listed below.

You may have noticed that Japanese beetles often attack one plant severely, leaving a lucky neighbor relatively unscathed. Apparently, when beetles initiate an attack, specific odors are released by the damaged plant. These send a signal to other beetles something like "good food, eat here”. This foliar attractant is compounded when female beetles release a chemical message called a sex pheromone. The sex pheromone says to the guy beetles, “how'd you like to spend a little time with me?” A rambunctious love fest and feeding frenzy erupt, and, in the process, your plant takes a beating.

Japanese beetle traps catch thousands of beetles. 

Clever chemists have been able to synthesize both the floral attractant and the sex pheromone and place them in a lure. Attach the lure to a few plastic fins for beetles to bump into, and a funnel to direct them into a plastic bag and, voilà; you have a Japanese beetle trap. Japanese beetle traps capture beetles by the thousands, but they may not be all that effective in protecting your plants. Plants near the traps may actually sustain more damage, as beetles lured to the vicinity mill around and chew on your plants as they await their turn to hit the fins and be captured. It is best to place these traps far away from valued plants you want to protect.

Japanese beetle grubs like this one thrive on roots of grasses. 

If Japanese beetles are a chronic problem in your garden or landscape, the best way to get relief may be to reduce the numbers breeding in your lawn, especially if you have irrigated turf. One promising “green” approach is to apply insect pathogenic nematodes, tiny roundworms that attack and kill beetle grubs. Nematodes enter the grub and release a lethal bacterium. There are many different species and strains of nematodes. Dave Shetlar of the Ohio State University suggests that products containing strains of Steinernema carpocapsae are a bit less effective against beetle grubs than species in the clan named Heterorhabditis. You must wait until late July or August when grubs are in the soil, if you go with the nematode treatment route.

There are also several potent soil insecticides that can be applied in late July through August that are very effective in killing the tiny grubs as they hatch from eggs and feed near the soil surface. When using any insecticide, always read the label and follow the directions carefully. Many insecticides are available to control Japanese beetles on plants, but multiple applications may be necessary if you cannot tolerate damage by these critters.

Another nifty way to help reduce damage is to simply knock the beetles from your plants into a bucket of soapy water. If you do this early in the season of beetle misery when beetles are first observed, you may reduce the chemical cues that incite a feeding frenzy. Beetle removal may be most successful in early morning or late evening when beetles are less active. There is a strange kind of justice in drowning this pest in soapy water. Save the bodies of the little rascals captured in your bucket or trap. The earthly remains of so many beetles make a wonderful addition to a compost pile that can later be used to nourish your garden.

Flowers and fruit are a delightful breakfast for this Japanese beetle.